Re: [PATCH v3 1/2] hw/nvme: Implement shadow doorbell buffer support

[Guenter Roeck]

On 12/12/22 01:58, Klaus Jensen wrote:
> On Dec  8 12:39, Guenter Roeck wrote:
> > On Thu, Dec 08, 2022 at 12:13:55PM -0800, Guenter Roeck wrote:
> > > On Thu, Dec 08, 2022 at 10:47:42AM -0800, Guenter Roeck wrote:
> > > > > A cq head doorbell mmio is skipped... And it is not the fault of the
> > > > > kernel. The kernel is in it's good right to skip the mmio since the cq
> > > > > eventidx is not properly updated.
> > > > >
> > > > > Adding that and it boots properly on riscv. But I'm perplexed as to why
> > > > > this didnt show up on our regularly tested platforms.
> > > > >
> > > > > Gonna try to get this in for 7.2!
> > > >
> > > > I see another problem with sparc64.
> > > >
> > > > [    5.261508] could not locate request for tag 0x0
> > > > [    5.261711] nvme nvme0: invalid id 0 completed on queue 1
> > > >
> > > > That is seen repeatedly until the request times out. I'll test with
> > > > your patch to see if it resolves this problem as well, and will bisect
> > > > otherwise.
> > > The second problem is unrelated to the doorbell problem.
> > > It is first seen in qemu v7.1. I'll try to bisect.
> >
> > Unfortunately, the problem observed with sparc64 also bisects to this
> > patch. Making things worse, "hw/nvme: fix missing cq eventidx update"
> > does not fix it (which is why I initially thought it was unrelated).
> >
> > I used the following qemu command line.
> >
> > qemu-system-sparc64 -M sun4v -cpu "TI UltraSparc IIi" -m 512 -snapshot \
> >      -device nvme,serial=foo,drive=d0,bus=pciB \
> >      -drive file=rootfs.ext2,if=none,format=raw,id=d0 \
> >      -kernel arch/sparc/boot/image -no-reboot \
> >      -append "root=/dev/nvme0n1 console=ttyS0" \
> >      -nographic -monitor none
>
> Hi Guenter,
>
> Thank you very much for the detailed reports and I apologize for the
> fallout of this.
>
> I think this is related to missing endian conversions when handling the
> shadow doorbells. I'm not sure if there is a kernel issue here as well,
> because as far as I can tell, the shadow doorbells are updated like so:
>
>    old_value = *dbbuf_db;
>    *dbbuf_db = value;
>
> (where `value` is the new head/tail value depending on if this is an sq
> or cq).
>
> Keith, is the kernel doing something magically here I am not aware of,
> or isn't this missing some cpu_to_le32() / le32_to_cpu() calls as well?

Wouldn't that mean that nvme doorbell support would be broken in Linux
on all big endian systems ? Maybe it is, but it seems a bit unlikely.

Thanks,
Guenter